Protein kinases are involved in the signal transduction pathways linking growth factors, hormones and other cell regulation molecules to cell growth, survival and metabolism under both normal and pathological conditions. One such protein kinase, protein kinase B (also known as Akt), is a serine/threonine kinase that plays a central role in promoting the proliferation and survival of a wide range of cell types, thereby protecting cells from apoptosis (programmed cell death) (Khwaja, Nature 33-34 (1990)). Three members of the Akt/PKB subfamily of second-messenger regulated serine/threonine protein kinases have been identified and are termed Akt1/PKBα, Akt2/PKBβ, and Akt3/PKBγ. A number of proteins involved in cell proliferation and survival have been described as substrates of Akt in cells. Two examples of such substrates include glycogen synthase kinase-3 (GSK3) and Forkhead transcription factors (FKs). See Brazil and Hemmings, Trends in Biochemical Sciences 26, 675-664.
A number of protein kinases and phosphatases regulate the activity of Akt; For instance, activation of Akt is mediated by phosphatidylinositol 3-kinase (PI3-K), which initiates the binding of second messenger phospholipids to the pleckstrin homology (PH) binding domain of Akt. The binding anchors Akt to plasma membrane and results in phosphorylation and activation of the enzyme. Amplifications of the catalytic subunit of PI3-K, p110α, or mutations in the PI3-K regulatory subunit, p85α, lead to activation of Akt in several types of human cancer. (Vivanco and Sawyers, Nature Reviews in Cancer (2002) 2: 489-501.
The tumor suppressor, PTEN, is a critical negative regulator of Akt activation by PI3-K. Myers et al. Proc. Nat. Acad. Sci. 95, USA (1998) 13513-13518. Inactivating mutations in the Pten gene have been found at high frequencies in a large number of human tumors and tumor cell lines, including prostate cancer, breast cancer, ovarian cancer, glioblastoma, melanoma and other cancer types. Inactivation of the PTEN protein results in elevated levels of phosphorylated Akt and increased Akt activity in tumor cells. Li, et al., Science (1997) 275: 1943-1947; Guldberg, et al., Cancer Research (1997) 57: 3660-3663; Risinger, et al., Cancer Research (1997) 57: 4736-4738; Vivanco and Sawyers, Nature Reviews in Cancer (2002) 2: 489-501. In addition to overactivation of Akt due to defects in PTEN, direct amplication and/or overexpression of Akt2 and Akt3 have been found in human neoplasia, for example ovarian, pancreatic, prostate and breast cancer cells (Cheung et al., Proc. Nat. Acad. Sci. USA (1992) 89:9267-9271; Cheung et al., Proc. Nat. Acad. Sci. USA (1996) 93:3636-3641; Nakatani et al., J. Biol. Chem. (1999) 274:21528-21532).
The critical role of Akt in cell proliferation and survival is further strengthened by studies showing that germline knockout of Akt1 results in partial embryonic lethality. The surviving littermates display stunted growth, increased organismal apoptosis, and early deaths. (Cho et al., J. Biol. Chem. (2001) 276: 38349-38520; Chen et al., Genes Dev. (2001) 15: 2203-2208). It has also been demonstrated that pharmacological inactivation of Akt induces apoptosis in cultured human ovarian cancer cells (Yuan et al., Oncogene 19, 2324-2340, 2000) and decreases growth of a human ovarian carcinoma xenograft in mice (Hu et al., Clin. Cancer Res. 6, 880-886, 2000).
Recent studies have also demonstrated the role of the PI3-K/AKT pathway in the life cycle of numerous viruses. Some viral proteins have been shown to directly activate the PI3-K/Akt pathway, thus providing an environment favorable for viral replication. These include the Tat protein of human immunodeficiency virus (HIV), Protein X of hepatitis B virus, and NS5A of hepatitis C virus (Borgatti et al., Eur. J. Immunol. (1997) 27: 2805-2811; Lee et al., J. Biol. Ciem. (2001) 276: 16969-16977; He et al., J. Virol. (2002) 76: 9207-9217). The PI3-K/Akt pathway is also required for initiation and completion of the replication cycle of human cytomegalovirus (HCMV). In fact, pharmacological inactivation of this pathway results in abortive production of HCMV and survival of the host cells (Johnson et al., J. Virol. (2001) 75: 6022-6032).
Because of its pivotal role in the regulation of cell survival, Akt provides a novel therapeutic target for the effective treatment of various disorders, particularly cancer and viral infections. However, such treatment requires the development of potent, selective inhibitors of Akt. Thus, the present invention provides a class of novel inhibitors of Akt, compositions comprising these compounds, and methods of using the compounds.